Performance and efficiency of internal combustion engines are affected by the characteristics of their exhaust systems. When designing an exhaust system, compromises must be made concerning engine performance at different speeds. For example, the exhaust system may be designed so that the engine will operate at maximum efficiency for maximum power and fuel economy at high engine speeds, at the sacrifice of performance and efficiency at low to medium engine speeds. Uneven idling also may result. For a static exhaust system, pressure downstream of the engine will vary with engine speed. Low or negative pressure helps to suck exhaust gas from the combustion chamber and, during valve overlap, to draw in a fresh fuel-air mixture. On the other hand, high positive pressure interferes with expulsion of exhaust gas from the engine and, during valve overlap, interferes with the incoming air-fuel mixture.
Particularly for high performance engines in which exhaust systems are designed to minimize pressure during valve overlap at high engine speeds for maximum power, exhaust systems have been proposed with special mufflers having throttle valves to restrict exhaust flow through the mufflers at low engine speeds in order to lessen or eliminate "back pressure." known systems are of complicated construction and/or have mechanical components that produce irritating rattles.